Double crucible Czochralski crystal growth apparatus

ABSTRACT

An apparatus useful for double crucible Czochralski crystal growth comprises an inner crucible fixed within an outer crucible wherein the inner crucible contains an extra volume or reservoir of semiconductor melt when flow of semiconductor melt from the outer crucible into the inner crucible through means interconnecting the crucibles ceases.

TECHNICAL FIELD

This invention is directed to a method of producing semiconductor singlecrystal material using the Czochralski process. In particular, themethod employs the use of a first and second crucible having aninterconnecting channel to permit the flow of semiconductor melttherebetween.

BACKGROUND OF THE INVENTION

It is well known to produce semiconductor single crystal material usingthe Czochralski technique by forming a melt of the crystal material andbringing a seed crystal into contact with the melt. The seed is thenpulled slowly upwards, the molten material solidifying at the seed-meltinterface, thus forming a single crystal billet as the seed is continuedto be pulled slowly upwards. Alternatively, of course, the billet can beformed by maintaining the seed in a fixed position while slowly loweringthe melt relative to the position of the seed.

While such a method has been found to be quite effective, the crystalproduced suffers from non-uniform electrical resistivity along itslength. This is primarily due to the fact that the doping agents (e.g.,arsenic, antimony, gallium or indium) commonly added to the puresemiconductor material (e.g., silicon, germanium), are more soluble inthe liquid semiconductor material than in the solidifying or solidcrystal. Hence, in a growing crystal, the concentration of doping agentin the melt keeps increasing as the crystal is pulled from the melt.This steadily increasing concentration of doping agent remaining in themelt also results in an increase of dopant and hence a decrease inresistivity along the length of the grown crystal billet as well aseventual saturation of the dopant in the melt. This saturation thenresults in the precipitation of a separate phase from the melt, which inturn provides nucleation sites for polycrystalline growth, therebyinterfering with the continued, desired, single crystal growth. Theproblem of saturation and polycrystalline formation is especiallysignificant in the growth of a heavily doped crystal wherein the dopanthas a high segregation behavior.

The aforementioned problems have been addressed by a technique known asthe double crucible method for Czochralski crystal growth. Generally,this method of pulling crystals from a melt employs an inner cruciblefrom which the crystal is pulled, which inner crucible is positionedwithin an outer crucible containing a reservoir of material supplied tothe inner crucible through an orifice connecting the two crucibles. Thisdouble crucible technique is employed to control the dopantconcentration of the melt by, for example, providing a melt ofrelatively high concentration of dopant in the inner crucible and one oflesser dopant concentration or dopant-free material in the outercrucible. As the crystal is being pulled from the inner crucible, thelower concentration material of the outer crucible passes through theorifice connecting the crucibles and enters the inner crucible therebymaintaining a uniform dopant concentration in the inner crucible. Thismethod is specifically suitable for the growth of crystals wherein thedopant is significantly more soluble in the melt than in the solid growncrystal. An example of such a crystal is antimony doped silicon. InCzochralski growth of such a crystal, as the crystal is being pulledfrom the melt, only a small percentage of the dopant enters the growingcrystal while the remaining melt normally would tend to become more andmore concentrated with dopant. However, with the use of a reservoir inthe outer crucible which consists of a melt containing a low dopantconcentration or a dopant-free melt which enters the inner cruciblethrough the connecting orifice and replaces the amount of melt removedfrom the crucible by the growing crystal, one can approximately maintainthe initial concentration of dopant in the inner crucible melt.

Notwithstanding the advantages gained by the use of double crucibletechniques as opposed to the original single crucible Czochralski growthtechnique, it has been found that in the growth of certain crystals,such as heavily antimony doped silicon, e.g., on the order of 4×10¹⁸atoms/cc, that while a relatively uniform resistivity of the billet canbe achieved, typically only about 50-60% of the initial total charge ofmelt material can be pulled into a single crystal before polycrystallinegrowth is observed. It is therefore desirable to provide a method forpulling crystals of this kind in which substantially more than 50-60% ofthe initial charge is useful, thereby allowing the growth of largercrystals and further increasing the efficiency and economics of thecrystal growth process. I have discovered that an important parameter indealing with the aforementioned problem of limited useful melt charge isthat of the relative ratios of the volumes of the inner crucible to theouter crucible as compared to the ratios of the cross-sectional areas ofthese crucibles. In the prior art double crucible apparatus, the ratioof the primary areas of the inner and outer crucibles as compared to theratio of the volumes of the respective crucibles are generally eitheridentical or the area ratio is greater than the volume ratio. This canbe seen, for example, with reference to U.S. Pat. No. 2,892,739.

SUMMARY OF THE INVENTION

The instant apparatus overcomes the foregoing problem by incorporatingmy discovery that if an "extra volume" of melt is provided in the innercrucible, as much as 75-80% of the total melt can be employed beforepolycrystalline growth is observable. The instant apparatus comprises aninner crucible and an outer crucible such that the ratio of the volumeof the inner crucible to the volume of the outer crucible is greaterthan the ratio of the primary cross-sectional areas of the respectivecrucibles. As previously stated, in conventional double crucibledesigns, the ratio of the volume of the melt of the inner crucible tothat of the outer crucible is generally equal to or sometimes less thanthe ratios of the areas of the respective crucibles.

It should be noted that the term inner crucible and outer crucible areused for convenience so as to relate the subject invention to the priorart. The inner crucible in actuality need not lie fixed or floatingwithin the melt of the outer crucible but may be separated from theouter crucible provided an interconnection exists between what is termedthe outer crucible and the inner crucible so as to allow melt to flowfrom the outer crucible to the inner crucible in a quantity essentiallysufficient to replace the melt lost from the inner crucible by thegrowing crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-sectional view depicting a typical priorart double crucible apparatus used in Czochralski crystal growth;

FIGS. and 2, 3 and 4 are elevational views depicting alternative doublecrucible apparatus in accordance with the instant invention; and

FIG. 5 is a graphical representation comparing the percent of meltsolidified before polycrystalline growth for various crucibleconfigurations with respect to dopant concentration.

DETAILED DESCRIPTION

The instant invention will be described with reference to particularexemplary embodiments, each of which have one commonality. That is, eachof the embodiments shown employ first and second chambers for containingmolten semiconductor material which chambers are interconnected with acommunicating channel therebetween so that the melt from the secondchamber flows into the first chamber and wherein the crystal is pulledfrom the first chamber. Another commonality of each of the embodimentsis that the first chamber contains what is termed herein an "extravolume." Another way to describe this extra volume is where the ratio ofthe volume of the first chamber to that of the second chamber is greaterthan the ratio of the primary area of the first chamber to the primaryarea of the second chamber. It should be understood that the particularembodiments shown are for purposes of exposition and not for limitation.Various other embodiments are contemplated which also maintain theaforesaid commonality.

The extra volume referred to is that volume which remains in the innercrucible after exchange of melt material from the outer crucible is nolonger possible. It is generally the volume or capacity of the innercrucible which lies below the interconnection tube which provides achannel of flow between the crucibles.

Referring to FIG. 1, there is depicted a prior art double crucibleCzochralski semiconductor crystal growing apparatus, generally indicatedby the numeral 10. The crystal apparatus 10 is comprised of a housing 11(e.g., graphite) surrounded by a series of high frequency inductioncoils 12. An outer crucible 13 is seated within the housing 11 and aninner crucible 14 is shown suspended in a first semiconductor melt 16within the outer crucible 13. A second melt 17 is present within theinner crucible 14. The first melt 16 within the outer crucible 13 is amelt of semiconductor material having a first dopant concentration leveland the second melt 17, within the inner crucible 14, is a melt of thesame semiconductor material having a second dopant concentration level.The inner crucible 14 is fixedly mounted within the outer crucible 13 bymeans of supports 19. When growing crystals, such as antimony dopedsilicon, in which the dopant is much more soluble in the melt than inthe pulled crystal, the dopant concentration level in the first melt 16in the outer crucible 13 may vary from zero to a level which is lowerthan the dopant concentration level of the second melt 17 in the innercrucible 14. An orifice or channel 18 is provided in the bottom of theinner crucible 14 so as to permit flow of the melt 16 into the innercrucible 14. It should be observed that the inner and outer cruciblesare typically essentially uniformly cylindrical in shape such that theratio of the volume of the inner crucible to the outer crucible is lessthan the ratio of the cross-sectional areas of the inner crucible to theouter crucible cross-sectional area. If the length of the inner cruciblewere extended to the bottom of the outer crucible, and the orifice wasat or near the bottom of the inner crucible, the aforementioned ratioswould essentially be equal. These relationships can be shownmathematically as follows: Where V_(o) represents the volume of the meltin the outer crucible; V_(i) represents the volume of the melt in theinner crucible; r_(o) represents the radius of the outer crucible; r_(i)represents the radius of the inner crucible; h_(o) represents the heightof the melt within the outer crucible; h_(i) represents the height ofthe melt within the inner crucible; and where the thickness of the innercrucible walls are small as compared to r_(i) and h_(i)

    V.sub.o =πr.sub.o 2h.sub.o -πr.sub.i 2h.sub.i

    V.sub.i =πr.sub.i 2h.sub.i

    A.sub.o =πr.sub.o 2-πr.sub.i.sup.2

    A.sub.i =πr.sub.i.sup.2 ##EQU1## Where the inner crucible extends to or near the bottom of the outer crucible h.sub.i ≈h.sub.o and

    V.sub.i /V.sub.o =A.sub.i /A.sub.o

In the case where h_(i) <h_(o) (and r_(i) <r₂), that is where the innercrucible height is less than the outer crucible height,

    V.sub.i /V.sub.o <A.sub.i /A.sub.o.

In operation, a rotating pull rod 21 holding a single crystalsemiconductor seed 22 causes the seed to contact the surface of the melt17 in the inner crucible 14 and the pull rod 21 is moved upward to drawa single crystal semiconductor billet 23 from the melt 17. As the singlecrystal semiconductor billet 23 is being drawn from the inner cruciblemelt 17, portions of the outer crucible melt 16 pass through the opening18 into the inner crucible 14 to (1) replenish the melt used to form thecrystal billet 23 and (2) maintain the dopant concentration in the melt17 substantially constant by diluting the higher dopant concentration inthe crucible melt 17 with the lower concentation melt 16 in the outercrucible.

Although the above process operates effectively, it has been found usingprior art fixed crucible configurations that only 50-60% of the melt canbe used before polycrystalline growth occurs.

FIGS. 2, 3 and 4 depict a crystal growing apparatus exemplary of theinstant invention. In the embodiment shown in FIG. 2 the double crucibleapproach to Czochralski crystal growth is applied without the use of aninner and outer crucible but rather with two separate crucibles whichare analogous to the inner and outer crucibles. While the apparatusshown in FIG. 2 may not be the most practical apparatus for employingthe novel invention, it is a viable apparatus and serves as an easyillustration of the instant invention. The apparatus as shown in FIG. 2comprises a first crucible 30 which is analogous to the inner cruciblein the prior art double crucible growth apparatus and a second crucible31 which is analogous to the outer crucible of the prior art doublecrucible growth apparatus. The lower portion of crucible 31 is connectedto crucible 30 at a point above the lower portion of crucible 30 by aninterconnecting tube 32 which allows semiconductor melt material fromcrucible 31 to flow into crucible 30. Crucible 30 contains asemiconductor melt material 33 having a predetermined desired dopantconcentration while the semiconductor melt 34 contained in crucible 31contains no dopant or a dopant concentration less than that ofsemiconductor melt material 33. Gravity causes the melt material incrucibles 30 and 31 to reach the same level above the interconnect tube32. As the crystal is pulled in the usual fashion from the melt 33 incrucible 30, semiconductor melt material 33 in crucible 30 is depletedand semiconductor melt material 34 from crucible 31 flows throughinterconnecting tube 32 into crucible 30 to compensate for the loss ofmaterial in crucible 30 and maintaining an essentially evenconcentration of dopant within the semiconductor melt of crucible 30 bydiluting the melt 33 which would otherwise tend to become more and moreconcentrated in dopant as the crystal is pulled. When the level of meltmaterial in the crucibles falls to the level of the interconnecting tube32, flow of melt from crucible 31 into crucible 30 ceases. As can beseen in the design of the novel apparatus, a quantity or volume of meltstill remains in the crucible 30 from which the crystal is pulled. Thisvolume is termed the "extra volume" of the inner crucible. The extravolume is shown as that volume below the dotted line across crucible 30.It can be seen that in this instance the ratio of the volume of crucible30 to the volume of crucible 31 is greater than the ratio of therespective cross-sectional areas of the crucibles. Hence

    V.sub.i /V.sub.o >A.sub.i /A.sub.o

The crucible system depicted in FIG. 3 comprises an outer crucible 40,an inner crucible 41 positioned within and spaced from the walls of theouter crucible 40, and at least one interconnecting orifice 42,typically capillary in size, in the wall 43 of the inner crucible 41.The orifice 42 permits flow of semiconductor melt 44 contained in outercrucible 40 to inner crucible 41 as semiconductor melt 45 contained inthe inner crucible 41 is depleted by the formation of the single crystalbillet 23 pulled therefrom. The wall 43 of inner crucible 41, which isgenerally cylindrically shaped, extends downwardly through and past thebottom of the outer crucible 40. The orifice 42 is preferably locatedadjacent and immediately above the line where the bottom of the outercrucible 40 meets the wall 43 of the inner crucible 41. In thisconfiguration almost all of the melt 44 can pass from the outer crucible40 to the inner crucible 41 before flow ceases. The volume of the innercrucible 41 below the orifice 42 is the extra volume previously referredto. This is the volume of semiconductor melt remaining in the innercrucible when melt transfer from the outer crucible ceases. It canreadily be seen that in the embodiment depicted in FIG. 3, the ratio

    V.sub.i /V.sub.o >A.sub.i /A.sub.o

where V_(i) and A_(i) represent the volume and area respectively of theinner crucible and V_(o) and A_(o) represents the useful volume andprimary area of the outer crucible.

Another embodiment of the invention is depicted in FIG. 4. In thisembodiment there is also an outer crucible 50, an inner crucible 51 andat least one, preferably capillary size, orifice 52 interconnecting thecrucibles 50 and 51. The outer crucible 50 has a cylindrical cruciblewall 53 which extends beyond the bottom 54 of the outer crucible 50.This wall extension 55 is the outer wall of the lower portion 56 of theinner crucible 51 which has a larger diameter than the diameter of theupper portion 57 of the inner crucible 51. The inner crucible 51 isformed by an upper portion wall 58 which meets with and then becomescoextensive or shared with the bottom 54 of the outer crucible 50. Theinner crucible 51 is closed at its lower end by a bottom wall 59 whichlies below the bottom 54 of the outer crucible. The upper portion of theinner crucible wall 58 is spaced from and centered within the outercrucible wall 53. The orifices 52, as shown, are located in the bottomwall 54 and allow for flow of semiconductor melt 60 in the outercrucible into the lower portion of the inner crucible 51. Alternatively,the orifice may be in the vertical portion of the inner crucible wall,preferably at a point or points where the vertical wall becomescoextensive with the bottom of the outer crucible.

In this configuration, the primary area of the inner crucible is thatarea above the orifice 52. The primary area of the outer crucible is, asis the case in the embodiment of FIG. 2, the area of the outer crucibleadjacent the inner crucible primary area, that is, the differencebetween πr_(o) ² and πr_(i) ² where r_(o) is the radius of the outercrucible and r_(i) is the radius of the inner crucible. The extra volumeof the inner crucible is the volume of the lower portion of the innercrucible (the portion below the bottom wall 54 of the outer crucible)since this is the volume remaining after flow of melt from the outercrucible ceases. Again, as can be seen,

    V.sub.i /V.sub.o >A.sub.i /A.sub.o

where volumes are total volumes and areas are primary areas.

It has been found that even heavily doped crystals, e.g., heavily Sbdoped Si, pulled from double crucible configurations such as shown inthe above-described novel embodiments where

    V.sub.i /V.sub.o >A.sub.i /A.sub.o

or where an extra volume exists, can result in use of as much as 80% ormore of the total semiconductor melt before encountering polycrystallinecrystal formation, thereby making the overall process more economical.

I have discovered that the benefit derived by the extra volume whereX<(1-x) can be expressed or shown mathematically from the followinggeneral equation ##EQU2## where:

C₁ (X) is the dopant concentration in the inner crucible at any point intime;

k is the segregation coefficient of the dopant between solid and melt;

R is the ratio of the primary area of the outer crucible (or equivalent)to the primary area of the inner crucible (or equivalent);

x is the ratio of the extra volume to the total melt volume of the twochambers;

X is the ratio of the solidified or billet volume to the total meltvolume;

C₁ ^(i) is the initial dopant concentration in the inner crucible; and

C₂ is the dopant concentration in the outer crucible.

In the special situation where there is no dopant in the outer crucible,i.e., C₂ =0, then ##EQU3## Also when X>(1-x) the governing expression is##EQU4##

Utilizing the above equations, FIG. 5 has been determined for normalsingle crucible Czochralski crystal growth, normal straight wall doublecrucible (no extra volume) Czochralski crystal growth and novel extravolume apparatus Czochralski crystal growth. The Figure is a plot ofpercent melt solidified before polycrystalline growth against the dopantconcentration in the melt plotted as the multiple of the initial dopantconcentration. The particular plots were made for an antimony dopedsilicon melt used for the growth of a 0.013 ohm-cm. silicon crystalwherein the graph for normal double crucible growth is based upon anominal 10 inch diameter outer crucible and a 6 inch diameter innercrucible, where R=1.786. The graphs for the novel extra volume growthare based upon a 20% extra volume using a double crucible design asshown in FIG. 3. In one instance, the graph represents a plot for adouble crucible having a nominal 10 inch outer crucible diameter and a 6inch inner crucible diameter and in the other instance the graphrepresents a plot for a double crucible having a 10 inch outer cruciblediameter and a 6.5 inch inner crucible diameter where R=1.367.

It can readily be seen from the graphs that with a 20% extra volume asdefined herein, approximately 80% of the total melt is useful beforesaturation of silicon with antimony, and hence, before the growth ofpolycrystalline silicon.

What is claimed is:
 1. An apparatus for growing a single crystal from amelt comprising an inner crucible and an outer crucible, said innercrucible placed concentrically inside and at least a top portion thereofspaced from the walls of said outer crucible, means for allowing theflow of melt from said outer crucible to said inner crucible, said innercrucible having a reservoir for holding melt in said inner crucibleafter flow of melt from said outer crucible ceases.
 2. The apparatusrecited in claim 1 wherein said means for allowing flow of melt from onecrucible to the other is an orifice which is capillary in size.
 3. Theapparatus recited in claim 1 including means for heating said crucibles.4. The apparatus recited in claim 1 wherein said inner crucible extendsbelow the bottom of said outer crucible and wherein said means forproviding flow of melt from said outer crucible to said inner crucibleis at least one orifice located in the wall of said inner crucibleadjacent and above where said wall meets the bottom of said outercrucible.
 5. An apparatus for growing single crystals from a meltcomprising an inner crucible and an outer crucible, said inner crucibleplaced essentially concentrically inside said outer crucible and atleast a top portion thereof spaced from the walls of said outercrucible, means for allowing the flow of melt from said outer crucibleto said inner crucible, said inner crucible having a reservoir forholding melt in said inner crucible after flow of melt from said outercrucible ceases and wherein the lower portion of the wall of said innercrucible extends outwardly to meet the wall of said outer crucible suchthat the bottom of said outer crucible is formed by said lower portionof the wall of said inner crucible and wherein said inner crucibleextends below the bottom of said outer crucible.